Enhancements

Fix exporting broken configuration file in some occasions.
Fix directly treating not-exist ruleset file as URL.
Fix group fallback not working on Quantumult X configurations.
Fix typo in example profile.
Add specifying default external configuration file to use when none is provided.
Optimize codes.

base/pref.ini

@@ -18,6 +18,9 @@ exclude_remarks=(到期|剩余流量|时间|官网|产品)
 ;Only include nodes which remarks match the following patterns. Supports regular expression.
 ;include_remarks=V3.*港
 
+;Setting an external config file as default when none is specified, supports local files/URL
+;default_external_config=config/example_external_config.ini
+
 ;Clash config base used by the generator, supports local files/URL
 clash_rule_base=base/all_base.tpl